This invention relates to a magnetic inspection method or magnetic flaw detecting method and an apparatus therefor, and more particularly to a method and an apparatus for detecting flaws present on or in the inner surface of a steel pipe having a small diameter and a considerable length.
The inventors have developed a magnetic inspection apparatus for detecting flaws present on the outer surface of slub or round steel, or flaws present on the outer or inner surface of a steel pipe, and filed many patent and utility model applications therefor.
The principle of these apparatuses in such that a strong magnetic field is produced on a surface to be inspected, by an exciting magnet, and the magnetism-sensitive element is placed in the magnetic field thus produced for detecting a magnetic flux leaking from the position of a flaw present on the surface of a material to be inspected.
The methods for detecting flaws in the inner surface of a steel pipe are classified into two types, i.e., one, in which a head incorporating an exciting magnet mechanism and a detecting mechanism therein is inserted into a pipe along the inner surface of the pipe, and another, in which the head similar thereto is moved along the outer surface of the pipe. These prior art methods or apparatuses can not avoid more or less shortcomings. In other word, the former dictates the insertion of a head into a pipe, thus finding applications only for pipes of a large diameter (over 300 mm in inner diameter), while the latter may be applied to the inspection for pipes having relatively small diameters, while suffering from poor detecting capability.